Field of the Invention
The present invention relates to a fail-safe apparatus for an inverter.
Description of the Related Art
In recent years, due to the tendency toward gasoline mileage regulation on automobiles, a mild-hybrid automobile and the like have been put to practical use; For example, a synchronous motor equipped with an inverter is utilized in such a hybrid automobile. In addition, due to a demand for a high-power motor and the like, a system utilizing a new DC power source such as a 48-V battery or a 48-V lithium ion battery has been drawing attention, in comparison with a system utilizing 12-V battery DC power source. The foregoing synchronous motor includes a synchronous motor that utilizes a permanent magnet for producing a magnetic field and operates at a high rotation speed. While such a synchronous motor equipped with a permanent magnet does not require any excitation current, the induction voltage produced by the permanent-magnet magnetic field increases in proportion to the rotation speed. Due to this phenomenon, when the rotation speed of the motor becomes so high as to exceed a given value, the produced induction voltage exceeds the output voltage of the inverter; therefore, in the control implemented at a time when the motor rotates at a high speed, there is utilized a method in which the output voltage of the inverter is suppressed by use of weak-magnetic-field control or the like utilizing PWM (Pulse Width Modulation)-control.
The foregoing method is applied also to the case where electric power is generated by use of a synchronous motor equipped with a magnetic-field permanent magnet; because when the rotation speed of the motor becomes so high as to exceed a given value, the produced induction voltage exceeds the DC power source voltage of the inverter, there is utilized a method in which weak-magnetic-field control utilizing PWM-control is implemented even at a time when electric power is generated so that the generated voltage is suppressed from exceeding the upper limit value of the restricted voltage.
However, in the case of such a motor, as described above, that utilizes a permanent magnet for producing a magnetic field and operates at a high rotation speed, when an abnormality occurs in the inverter at a time when the motor rotates at a high rotation speed, the induction voltage produced by the rotation of the motor becomes excessively high; thus, the excessively high voltage may break the inverter; in addition, for example, in the case of a 48-V system, the induction voltage exceeds the upper limit voltage of the DC power source and hence the voltage may increase up to a value that is so high as to provide danger to a human body.
Accordingly, as a suppression method at a time when an abnormality occurs in the inverter and hence the induction voltage of the motor produces an excessive voltage, there has been proposed a configuration in which when the inverter is shut off, the inverter is controlled so as to be in a three-phase short-circuiting mode (refer to Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2011-172343
Use of the configuration disclosed in Patent Document 1 makes it possible that when due to an abnormality in the inverter, the inverter is shut off and the voltage produced by the induction voltage of the motor becomes the same as or higher than a predetermined voltage, the inverter is made to be in the three-phase short-circuiting mode so that the voltage can be suppressed from becoming excessively high. However, in the case of the foregoing configuration, the inverter cannot be controlled to be in the three-phase short-circuiting mode for the excessive voltage that is produced when the terminal of the motor to be connected with a DC power source is disconnected at a time when the motor is rotating at a high rotation speed and weak control is required or that is produced through a contributing factor other than the shutoff of the inverter, such as a failure in a sensor.